[PATCH v2 2/4] iommu/iova: Avoid double-negatives in magazine helpers

[John Garry]

A similar crash to the following could be observed if initial CPU rcache
magazine allocations fail in init_iova_rcaches():

Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
Mem abort info:
   ESR = 0x96000004
   EC = 0x25: DABT (current EL), IL = 32 bits
   SET = 0, FnV = 0
   EA = 0, S1PTW = 0
Data abort info:
   ISV = 0, ISS = 0x00000004
   CM = 0, WnR = 0
[0000000000000000] user address but active_mm is swapper
Internal error: Oops: 96000004 [#1] PREEMPT SMP
Modules linked in:
CPU: 11 PID: 696 Comm: irq/40-hisi_sas Not tainted 5.9.0-rc7-dirty #109
Hardware name: Huawei D06 /D06, BIOS Hisilicon D06 UEFI RC0 - V1.16.01 03/15/2019
Call trace:
  free_iova_fast+0xfc/0x280
  iommu_dma_free_iova+0x64/0x70
  __iommu_dma_unmap+0x9c/0xf8
  iommu_dma_unmap_sg+0xa8/0xc8
  dma_unmap_sg_attrs+0x28/0x50
  cq_thread_v3_hw+0x2dc/0x528
  irq_thread_fn+0x2c/0xa0
  irq_thread+0x130/0x1e0
  kthread+0x154/0x158
  ret_from_fork+0x10/0x34

Code: f9400060 f102001f 54000981 d4210000 (f9400043)

 ---[ end trace 4afcbdfc61b60467 ]---

The issue is that expression !iova_magazine_full(NULL) evaluates true; this
falls over in in __iova_rcache_insert() when we attempt to cache a mag
and cpu_rcache->loaded == NULL:

if (!iova_magazine_full(cpu_rcache->loaded)) {
	can_insert = true;
...

if (can_insert)
	iova_magazine_push(cpu_rcache->loaded, iova_pfn);

As above, can_insert is evaluated true, which it shouldn't be, and we try
to insert pfns in a NULL mag, which is not safe.

To avoid this, stop using double-negatives, like !iova_magazine_full() and
!iova_magazine_empty(), and use positive tests, like
iova_magazine_has_space() and iova_magazine_has_pfns(), respectively; these
can safely deal with cpu_rcache->{loaded, prev} = NULL.

Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
---
 drivers/iommu/iova.c | 29 +++++++++++++++++------------
 1 file changed, 17 insertions(+), 12 deletions(-)

diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index 81b7399dd5e8..1f3f0f8b12e0 100644
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -827,14 +827,18 @@ iova_magazine_free_pfns(struct iova_magazine *mag, struct iova_domain *iovad)
 	mag->size = 0;
 }
 
-static bool iova_magazine_full(struct iova_magazine *mag)
+static bool iova_magazine_has_space(struct iova_magazine *mag)
 {
-	return (mag && mag->size == IOVA_MAG_SIZE);
+	if (!mag)
+		return false;
+	return mag->size < IOVA_MAG_SIZE;
 }
 
-static bool iova_magazine_empty(struct iova_magazine *mag)
+static bool iova_magazine_has_pfns(struct iova_magazine *mag)
 {
-	return (!mag || mag->size == 0);
+	if (!mag)
+		return false;
+	return mag->size;
 }
 
 static unsigned long iova_magazine_pop(struct iova_magazine *mag,
@@ -843,7 +847,7 @@ static unsigned long iova_magazine_pop(struct iova_magazine *mag,
 	int i;
 	unsigned long pfn;
 
-	BUG_ON(iova_magazine_empty(mag));
+	BUG_ON(!iova_magazine_has_pfns(mag));
 
 	/* Only fall back to the rbtree if we have no suitable pfns at all */
 	for (i = mag->size - 1; mag->pfns[i] > limit_pfn; i--)
@@ -859,7 +863,7 @@ static unsigned long iova_magazine_pop(struct iova_magazine *mag,
 
 static void iova_magazine_push(struct iova_magazine *mag, unsigned long pfn)
 {
-	BUG_ON(iova_magazine_full(mag));
+	BUG_ON(!iova_magazine_has_space(mag));
 
 	mag->pfns[mag->size++] = pfn;
 }
@@ -905,9 +909,9 @@ static bool __iova_rcache_insert(struct iova_domain *iovad,
 	cpu_rcache = raw_cpu_ptr(rcache->cpu_rcaches);
 	spin_lock_irqsave(&cpu_rcache->lock, flags);
 
-	if (!iova_magazine_full(cpu_rcache->loaded)) {
+	if (iova_magazine_has_space(cpu_rcache->loaded)) {
 		can_insert = true;
-	} else if (!iova_magazine_full(cpu_rcache->prev)) {
+	} else if (iova_magazine_has_space(cpu_rcache->prev)) {
 		swap(cpu_rcache->prev, cpu_rcache->loaded);
 		can_insert = true;
 	} else {
@@ -916,8 +920,9 @@ static bool __iova_rcache_insert(struct iova_domain *iovad,
 		if (new_mag) {
 			spin_lock(&rcache->lock);
 			if (rcache->depot_size < MAX_GLOBAL_MAGS) {
-				rcache->depot[rcache->depot_size++] =
-						cpu_rcache->loaded;
+				if (cpu_rcache->loaded)
+					rcache->depot[rcache->depot_size++] =
+							cpu_rcache->loaded;
 			} else {
 				mag_to_free = cpu_rcache->loaded;
 			}
@@ -968,9 +973,9 @@ static unsigned long __iova_rcache_get(struct iova_rcache *rcache,
 	cpu_rcache = raw_cpu_ptr(rcache->cpu_rcaches);
 	spin_lock_irqsave(&cpu_rcache->lock, flags);
 
-	if (!iova_magazine_empty(cpu_rcache->loaded)) {
+	if (iova_magazine_has_pfns(cpu_rcache->loaded)) {
 		has_pfn = true;
-	} else if (!iova_magazine_empty(cpu_rcache->prev)) {
+	} else if (iova_magazine_has_pfns(cpu_rcache->prev)) {
 		swap(cpu_rcache->prev, cpu_rcache->loaded);
 		has_pfn = true;
 	} else {
-- 
2.26.2